Electronic signaling systems are commonly “single-ended” or “differential.” Single-ended systems represent data as a varying voltage on a single conductor, whereas differential systems transmit information as complementary signals on separate conductors. Differential signaling is more noise tolerant, and can thus support higher speed performance, than single-ended signaling. The main advantage of single-ended over differential signaling is that fewer conductors are needed to transmit multiple signals.
The density of integrated circuits (ICs) has risen markedly faster than the number of pads available to convey external signals. As a result, many ICs are pad limited, which is to say that the area and cost of ICs are largely determined by the requisite number of external signaling pads. Single-ended signaling requires fewer pads and is therefore desirable for many pad-limited applications. Single-ended signaling is also desirable for many applications in which circuit-board routing space is limited.
The superior noise tolerance of differential signals is due in part to the fact that two signal halves are measured relative to one another so that noise common to both signals can be rejected. In a single-ended system, the value of a measured signal depends upon a reference voltage, typically ground, within a system. Reference voltages can be generated locally, possibly in the vicinity of the receiver, but such reference voltages do not carry the same dynamic noise that distorts the information signal. This is particularly true where single-ended signals that share a common reference voltage are conveyed on separate channels and are therefore subject to disparate noise environments. The generation and delivery of reference voltages must therefore be carefully considered when implementing a high-performance single-ended signaling system.
Some systems improve the performance of single-ended systems by conveying a suitable reference voltage with bundles of signal wires. Noise and channel effects that impact the signal wires similarly impact the reference voltage and thus tend to cancel at the receiver. Unfortunately, provisioning for each additional reference voltage increases the requisite number of pads: as noted above, a lower pad count is often the reason for using single-ended signaling in the first place.